The Constitution of the United States: A Transcription
Article. IV.
Section. 4.
The United States shall guarantee to every State in this Union a Republican Form of Government, and shall protect each of them against Invasion; and on Application of the Legislature, or of the Executive (when the Legislature cannot be convened), against domestic Violence.

More

• Section 4: Obligations of the United States – Wikipedia
• Section 4. Obligations of United States to States – Findlaw

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